The invention relates to the field of resistive heaters.
Thermal Spray
Thermal spray is a versatile technology for depositing coatings of metals or ceramics. It includes systems that use powder as feedstock (e.g., arc plasma, flame spray, and high velocity oxy-fuel (HVOF) systems), and systems that use wire as feedstock (e.g., arc wire, HVOF wire, and flame spray systems).
Arc plasma spraying is a method for depositing materials on various substrates. A DC electric arc creates an ionized gas (a plasma) that is used to spray molten powdered materials in a manner similar to spraying paint.
Arc wire spray systems function by melting the tips of two wires (e.g., zinc, copper, aluminum, or other metal) and transporting the resulting molten droplets by means of a carrier gas (e.g., compressed air) to the surface to be coated. The wire feedstock is melted by an electric arc generated by a potential difference between the two wires.
In flame spray, a wire or powder feedstock is melted by means of a combustion flame, usually effected through ignition of gas mixtures of oxygen and another gas (e.g., acetylene).
HVOF uses combustion gases (e.g., propane and oxygen) that are ignited in a small chamber. The high combustion temperatures in the chamber cause a concurrent rise in gas pressure that, in turn, generates a very high speed effluent of gas from an orifice in the chamber. This hot, high speed gas is used to both melt a feedstock (e.g., wire, powder, or combination thereof) and transport the molten droplets to the surface of a substrate at speeds in the range of 330–1000 m/sec. Compressed gas (e.g., compressed air) is used to further accelerate the droplets and cool the HVOF apparatus.
A thermal sprayed coating has a unique microstructure. During the deposition process, each particle enters the gas stream, melts, and cools to the solid form independent of other particles. When molten particles impact the substrate being coated, they impact (“splat”) as flattened circular platelets and freeze at high cooling rates. The coating is built up on the substrate by traversing the plasma gun apparatus repeatedly over the substrate building up layer by layer until the desired thickness of coating has been achieved. Because the particles solidify as splats, the resultant microstructure is very lamellar with the grains approximating circular platelets randomly stacked above the plane of the substrate.
Resistive Heaters
Thermal spray technology has been used to deposit a coating for use as a heater. A resistive heater produces heat by the collision of electrons with the atoms of the heater material. The rate at which heat is generated is the power, which depends on the amount of current flowing and the resistance to the current flow offered by the material. The resistance of a heater depends on a material property termed “resistivity,” and a geometric factor describing the length of the current path and the cross-sectional area through which the current passes.
Previously, resistive coatings have been deposited using thermal spray. In one such example, metal alloys such as 80% Nickel-20% Chrome are deposited and used as heaters. In another example, a metal alloy in powder form is mixed with powders of electrical insulators such as aluminum oxide prior to deposition. The blended material is then deposited using thermal spray to form a coating of resistive material. When nickel-chrome is deposited as a resistive heater, however, the bulk resistivity of the layer is still rather low, which makes it more difficult to form an element because a long path length is required to achieve a high enough resistance. When oxide-metal blends are deposited, large discontinuities in the composition of resistive layer, which produce variations in power distribution over a substrate, are frequently present.
Molding Thermoplastic Materials
Many plastic and metal parts are manufactured by injecting molten metal or polymer melt into a complex cavity cut into steel, for example, aluminum automobile transmission housings or polycarbonate computer cases. Injection-molding machinery melts a thermoplastic powder in a heating chamber and forces it into a mold, where it hardens. The operations take place at rigidly controlled temperatures and intervals. In an injection molding process, it is important to maintain a material such as polycarbonate in a molten state as it flows into and through a mold cavity space. Additionally, the shear stress profile of the flow of resin must be monitored and managed to insure proper filling of the cavity space. If the molten resin solidifies too rapidly when it encounters a cold mold, it will not penetrate narrow cavities and will form weak knit lines where two flows intersect. Accordingly, much effort has been directed towards improving heat management and flow control in the injection molding process.